A service I like to offer with the sale of some of my houses, is if there is no porch built, or it's a measly porch, we will try to get one built. It will be a part of the total price of the house, so it wouldn't be a problem for me.
Have you ever offered to do something like this yourself?
That's something you could totally do I suppose. I think that would lie with the homeowner over you, but if it's a service you want to offer, I say go for it.
I had a few buyers interested in a home before I finished it. I put it on the market while I was still doing updates and repairs in case someone wanted to buy it off me before i had finished to do their own final touches. A couple had requested that if I sold to them, that I would build a deck off the back of the house and install and above-ground pool alongside it. They paid for the wood and pool, I did the manual labor of it and they tacked on an additional 7 grand to the home when they bought it.